docs: def check_dimensional_vertical_coordinate(self, ds):
          '''
          Check units for variables defining vertical position are valid under
          CF.
  
          CF §4.3.1 The units attribute for dimensional coordinates will be a string
          formatted as per the udunits.dat file.
  
          The acceptable units for vertical (depth or height) coordinate variables
          are:
          - units of pressure as listed in the file udunits.dat. For vertical axes
            the most commonly used of these include include bar, millibar,
            decibar, atmosphere (atm), pascal (Pa), and hPa.
          - units of length as listed in the file udunits.dat. For vertical axes
            the most commonly used of these include meter (metre, m), and
            kilometer (km).
          - other units listed in the file udunits.dat that may under certain
            circumstances reference vertical position such as units of density or
            temperature.
  
          Plural forms are also acceptable.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: list
          :return: List of results
          '''
          ret_val = []
          z_variables = cfutil.get_z_variables(ds)
          #dimless_standard_names = [name for name, regx in dimless_vertical_coordinates]
          for name in z_variables:
              variable = ds.variables[name]
              standard_name = getattr(variable, 'standard_name', None)
              units = getattr(variable, 'units', None)
              positive = getattr(variable, 'positive', None)
              # Skip the variable if it's dimensionless
              if (hasattr(variable, 'formula_terms') or
                  standard_name in dimless_vertical_coordinates):
                  continue
  
              valid_vertical_coord = TestCtx(BaseCheck.HIGH, self.section_titles["4.3"])
              valid_vertical_coord.assert_true(isinstance(units, basestring) and units,
                                               "§4.3.1 {}'s units must be defined for vertical coordinates, "
                                               "there is no default".format(name))
  
              if not util.units_convertible('bar', units):
                  valid_vertical_coord.assert_true(positive in ('up', 'down'),
                                                   "{}: vertical coordinates not defining pressure must include "
                                                   "a positive attribute that is either 'up' or 'down'".format(name))
  
              # _check_valid_standard_units, part of the Chapter 3 checks,
              # already verifies that this coordinate has valid units
  
              ret_val.append(valid_vertical_coord.to_result())
  
          return ret_val